This invention relates generally to measurement of displacement. In particular, it relates to a method for using electromagnetic wave interference pattern changes to effect such results.
Displacement measuring devices form the backbone of all industrial and research dimensional measurement, as well as provide the basis for most transducers of force, pressure, and other variables. Mechanical multiplication, air pressure change, resistive, capacitive, and inductive/reactive techniques are most common and form the basis of virtually all systems in practical industrial use.
While present methods have progressed to a high state of refinement, several problem areas remain. First, all electrical transduction means in common use today produce analog varying output signals which must be electronically converted if a digital readout or computer is used. The conversion process introduces errors and complexity, and it has been the goal of considerable transducer research to develop practical, intrinsically digital or at least quasi-digital transduction means obviating the need for such analog to digital (A-D) conversion.
Another widespread problem is maintenance and calibration of present transducer systems. Time and environmental effects, together with amplifier drift, all cause heretofore constructed devices to lose their calibrated performance (eg. in a load cell, voltage output per pound of force applied) specification. Periodic recalibration is therefore necessary, a time consuming job in many cases.
When the "rezeroing" or calibration required by present displacement sensing systems cannot be performed, such devices are essentially worthless. For example, long term, accurate measurement of strain or displacement in structural members (bridges, dams, pipelines, aircraft etc.) is difficult, if not impossible using present techniques. Thus, in many instances, no practical, accurate means of long term monitoring of the safety of such structures has been possible heretofore.
Another displacement measurement problem with many present systems is erroneous readings caused by the necessity of the gaging system to contact the member whose displacement is desired. In addition, many applications virtually require that no contact be made, a case in point being displacements of vibrating members. For measurement of such displacements it is necessary to have a high degree of accuracy, sufficient frequency response, and it is highly desirable that no modification of any kind be made to the member measured. Capacitive, inductive and laser interferometer gages have been used, and none fulfills all requirements.
In view of the above difficulties, together with intrinsic non-linearities, susceptabilities to vibration, voltage fluctuations, stray electric fields, and the like, present electrical displacement transduction systems seldom have accuracies of more than one part in 200 of full scale range. Even in the case of resistance strain gage load cells (which are not displacement sensors per se), where most variables influencing their output can be compensated for over a small temperature range, the values of one part in 500 (of full scale load) often quoted are achieved only when frequent re-zeroing is possible--in many cases before each measurement.
Where dimensional measurement is performed over longer ranges, as in the control of machine tools, ever higher industrial requirements have necessitated installation of moire scale and laser interferometer apparatus. Because of the latter's expense, moire devices have been most used. Since their resolution depends on the number of graduations per unit length, such moire devices suffer manufacturing limitations on the length of fine-spaced (eg. 100 lines per mm) scales which can be obtained at reasonable cost. Even in the best versions, system accuracy is seldom greater than 3 microns per 30 cm.
It is an object therefore of this invention to provide a linear, quasi-digital, and extremely accurate non-contacting method and apparatus for sensing changes in the separation of two members.
It is also an object of this invention to provide a method and apparatus for sensing changes in separation which is capable of long term field use without frequent calibration or adjustment.
It is another object of the invention to provide means to determine changes in separation in two orthogonal directions.
It is yet another object of the invention to provide a means for accurately sensing displacements over ranges of one meter or more, without recourse to construction of finely spaced scales.
It is a further object of this invention to provide systems for the transduction of various quantities causing changes in the separation of two members.
Finally, it is an object of this invention to provide simple, inexpensive means for accomplishing the objects stated above.